Experiments were conducted to study the rate dependent transition to thermoacoustic instability in a turbulent afterburner rig. The afterburner rig contains v-gutters as flame stabilizers and simulates elevated inlet gas temperature using a preheater. Under quasi-
static increase in flow Reynolds number of the afterburner, screech is not observed; on the contrary, screech onsets when the Reynolds number is increased at a higher rate. Such a phenomenon is known as rate-induced tipping or R-tipping. When the Reynolds number is increased at a lower rate, screech appears as a few bursts of high-amplitude periodic oscillations amidst low-amplitude aperiodic oscillations, a state known as intermittency. As the rate of change of Reynolds number is increased, more bursts appear and with further increase in the rate, the proportion of periodic oscillations of the bursts increases in the time series of pressure fluctuations, approaching a state of limit-cycle oscillations. We show for the first time, a rate-dependent transition to thermoacoustic instability in a turbulent afterburner happening through intermittency, with respect to increasing the rate of change of Reynolds number. The onset of burst occurs earlier, i.e., bursts occurring at a lower Reynolds number as the rate of change
of Reynolds number is increased. The amplitude of bursts during intermittency is higher than that of the limit cycle oscillations. The rate dependent transition to screech through intermittency is analysed by studying the variation of probability density function (PDF) of the amplitude of pressure, variation of the amplitude of bursts, variation of the number of bursts of periodic oscillations, and the inlet conditions corresponding to theonset of bursts, with respect to different rates of change of Reynolds number. Rate dependent transition observed in the model afterburner suggests that combustion system
of a gas turbine engine should be subjected to different engine throttling rates to define the thermoacoustic stability map. We studied the coupled interaction between p' and q̇ ′ using measurements of global
unsteady heat release rate and acoustic pressure. As the Reynolds number is increased at a rate of 335 s −1 , the acoustic pressure transitions to sustained oscillations via bursts of periodic oscillations. The wavelet coherence plot provides information on the common spectral power which is an indicator of coupling strength and the phase relationship between the variables (known as phasors). We observe that the bursts of high coupling strength do not have the phasors aligned among them and the alignment increases as the number of bursts of high coupling strength increases. Secondly, we
observe the state of bursts of high coupling strength even after the acoustic pressure transitioned from a state of bursts of oscillations to a state of sustained oscillations. The p' attains a state of periodic oscillations (as observed in RPs and probabilities of
recurrence (P(τ)) plots) earlier i.e. at a lower Reynolds number than the q̇ ′ and therefore, much of the increase in the cross-correlation coefficient between the P(τ) of p' and q̇′ is because of the increase in the coupling of q̇′ with p'.

Further, we captured the flame dynamics using planar measurements of unsteady heat release rate. We observe that alternate vortex shedding and the associated dynamics of unsteady heat release rate is responsible for the sustenance of transverse thermoacoustic instability. Though the mechanism is the vortex shedding, we show that the vortex roll-
up itself does not generate acoustic energy. It is the flame ignition and extinction at the immediate wake of the flame holder that drives the thermoacoustic instability. The suppression techniques devised for disrupting the acoustic sources, cannot be misled by the vortex-flame roll up and requires a thorough investigation before implementing
such techniques. Also, we show that the patterns of the phasor field are a result of convection of reacting particles from the immediate wake to different zones of the combustor. Such an understanding can improve the modelling of the unsteady heat release rate of the thermoacoustic system.